Just doing that google bit. Atlas D does seem to be the key.
Included in the transfer was the 704 Strategic Missile Wing, which had been activated as USAF first missile wing on July 1, 1957. The 704th had a dual mission of training missile crews for other units and attaining an operation capability with the Atlas ICBM. It had one Atlas D squadron, the 576th Strategic Missile Squadron assigned. Activated on 1 April, the 576th had a dual responsibility of maintaining an Atlas D alert force and providing training for other SAC Atlas units.
the squadron was reactivated on 6 March 1958 as the 576th Strategic Missile Squadron (SMS) assigned to Cook AFB (later renamed Vandenberg AFB). On 1 April 1958, the 576th was assigned to the Strategic Air Command (SAC) as an Atlas Intercontinental Ballistic Missile (ICBM) unit, the nation's first ICBM unit. On 2 April 1966, the 576th was once again inactivated.
In January 1958, ARDC transferred the base to the Strategic Air Command (SAC). With facilities under construction for America’s first ICBM, on April 1, 1958, Headquarters SAC activated the 576th Strategic Missile Squadron. On October 4, 1958, Cooke AFB was renamed Vandenberg AFB in honor of the late General Hoyt S. Vandenberg, the Air Force's second Chief of Staff.
The first Atlas launcher to be completed (576A-1) was accepted from the contractor by the 1st Missile Division on October 16, 1958. The first Atlas D missile arrived the following February. Initially, the squadron’s Atlas D missiles were deployed at complexes 576A and 576B. Complex 576A consisted of three above-ground gantries; 576B had three above-ground coffin launchers of a type that would be constructed at other sites. Each complex had one launch control center.
The 576th SMS launched its first Atlas D on September 9, 1959. Immediately following the launch, SAC’s Commander in Chief, General Thomas S. Power declared Vandenberg’s Atlas missile operational. A month later, the squadron’s Atlas missiles were placed on an alert status. The activation had more psychological value than military value as the reliability of the Atlas D missile was highly questionable. Improved versions were already undergoing production along with launch facilities to support them. As the above-ground sites became operational, construction continued on a buried coffin launcher to hold an Atlas E missile (designated launch site 576C) and work began on two Atlas F silo lift launchers (576D and 5763).
By 1962, 11 prototype Atlas complexes had been constructed at Vandenberg AFB.
Project "Added Effort", the Air Force nickname for the programmed phaseout of all first-generation ICBMs, began on 1 May 1964 when the first Atlas D's were taken off alert at the 576th Strategic Missile Squadron
